The present invention is related to a service brake inspection system for maintaining the service brakes of an air brake system engaged while the parking or emergency brake system is also engaged to allow the operator to check slack adjustment of all service brakes, including those of a trailer, without assistance. More specifically, the system includes a manually-activated valve which is operable to maintain the service brakes of the vehicle engaged and further includes a safety feature which makes the system actuatable only when the emergency/parking brakes of the vehicle are also applied, so that the system cannot be activated while in the driving mode.